Ocular lenses are worn by many people to correct vision problems. Vision problems are caused by aberrations of the light rays entering the eyes. These include low order aberrations—such as myopia, hyperopia, and presbyopia and higher order aberrations—such as spherical aberration, coma, trefoil, and chromatic aberrations. Additionally, aberrations are introduced by the sphere, cylinder, and axis corrections for myopia, hyperopia, and presbyopia, respectively. The aforementioned aberrations significantly degrade the quality of the images on an image plane. Thus, removing introduced and natural aberrations increases visual acuity.
Typically in the prior art, ocular lenses are made by generating prescriptions in lens blanks. This is accomplished by altering the topography of the surface of the lens blank.
Recently, attention has been given to methods for generating prescriptions to correct low order aberrations in lens blanks using a patient's measured wavefront information. Currently, several techniques are utilized to determine the optimum low order refraction from measured high order aberrations, including: the Gaussian Least Squares Fit, point spread optimization, and neural network analysis. Some of these techniques may be employed to derive the best low order prescription based at least in part from measured high order aberration values and to “fit” an optimum wavefront across an entire spectacle lens based on the patient's measured wavefront.
Using one or more of these fitting techniques may yield a better refraction than conventional subjective refractions in the intermediate zone. Additionally, in many applications, consideration should be given to off-axis gaze angles. In particular, one disadvantage to traditional lens manufacturing is that many people experience distortion when looking off-center outside the central region, commonly called “swim.”
For example, distortion can be present in progressive addition lenses (PAL) that possess both far and near correction zones where the power change between the two zones is progressive. Due to the progressive power change, which is mostly due to changes in the front or back radius of curvature, there can be distortion around the near zone of the lens (swim). The progressive power change can create a channel of varying optical power and two swim zones adjacent to this channel. The power change in the channel can possess smooth transition and, in most instances, may not have any distortion. The swim zones can possess distortion due to off-axis astigmatism and other aberrations. The progressive design can be generated on the front side, which is typically cast molded, the back side, or on both sides. Additionally, PALs can be used by presbyopic patients to focus on objects that are far from the patient and on objects that are nearby without an abrupt change in power.
To solve distortion problems in PALs, the prior art methods determined a wavefront for a patient's spectacle lens based on the patient's measured wavefront to reduce distortion when the patient looks off-center outside a central zone of the spectacle. This is accomplished with a progressive addition surface (contour map) based on wavefront optimization and weighting functions that are independent of the lens blank base curves. The progressive addition surfaces may comprise a far zone, a near zone, an intermediate zone, and limiters for off-axis astigmatism.
Typically, the contour maps of the prior art have noticeable swim regions in spectacle lenses for some patients. Applying these contour maps on a lens leads to compromising the patient's visual acuity in one or more of the three power progression zones, i.e., far, near or intermediate. Thus, a method and apparatus are needed to provide customized progressive addition lenses (PALs) that lower perceptible distortions without significantly compromising visual acuity.
PAL designs for computer use typically include a very narrow far zone limited to 10-20 feet, a wide fixed intermediate zone to view the entire computer screen, and a fixed reading zone. Additionally, PALs for computer use are typically designed for a computer working distance of approximately 24 inches. In the prior art, the intermediate power is typically set to 50-60% of the reading power, representing the change in accommodation from 16″ to 24″. However, the change in accommodation from 16″ to 24″ varies from patient to patient. Additionally, the computer working distance for each individual changes due to variances in computer screen size and specific user preference. Thus, the prior art PALs do not function optimally.
Methods and apparatuses are needed to increase the visual acuity for specific users at their respective specific working distances.